Electrothermal heater mat

ABSTRACT

An electrothermal heater mat ( 3 ) is provided for an ice protection system for an aircraft ( 1 ) or the like. The heater mat ( 3 ) is a laminated heater mat and comprises dielectric layers ( 50 - 58 ), a heater element ( 501 ) and a temperature sensor ( 507 ). Each dielectric layer ( 50 - 58 ) comprises thermoplastic material, and the temperature sensor ( 507 ) comprises a sprayed metal track ( 5010, 5012 ) deposited on a substrate ( 50, 5019 ) comprising thermoplastic material. The substrate may be one of the dielectric layers ( 50 ) or a separate carrier ( 5019 ) which is smaller than the dielectric layers ( 50 - 58 ). The substrate ( 50, 5019 ) is laminated to at least a first one of the dielectric layers ( 53, 58 ) and the thermoplastic material of the substrate is (i) the same as the thermoplastic material of the first dielectric layer such that the thermoplastic material of the substrate is dispersed or merged into the thermoplastic material of the first dielectric layer or (ii) compatible with the thermoplastic material of the first dielectric layer such that the thermoplastic material of the substrate is fused to the thermoplastic material of the first dielectric layer. Thus, the formation of an undesirable discontinuity at the interface between the substrate ( 50, 5019 ) and the first dielectric layer ( 53, 58 ) is substantially prevented or minimized.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application represents the national stage entry of PCTInternational Application No. PCT/GB2011/000125 filed Jan. 31, 2011,which claims the benefit of Great Britain Application 1001581.6, filedJan. 29, 2010, both of which are hereby incorporated herein by referencefor all purposes.

FIELD OF THE INVENTION

The present invention relates generally to an electrothermal iceprotection system suitable for use in an aircraft or other aerodynamicstructure such as a blade of a wind turbine to prevent ice from formingand/or to remove ice that has already formed. These two functions may betermed anti-icing and de-icing, respectively.

BACKGROUND OF THE INVENTION

For an aircraft, the in-flight formation of ice on the external surfaceof the aircraft is undesirable. The ice destroys the smooth flow of airover the aircraft surface, increases drag and decreases the ability ofan aerofoil to perform its intended function.

Also, built-up ice may impede the movement of a movable control surfacesuch as a wing slat or flap. Ice which has built up on an engine airinlet may be suddenly shed in large chunks which are ingested into theengine and cause damage.

It is therefore common for aircraft, and particularly commercialaircraft, to incorporate an ice protection system. A commercial aircraftmay use a system which involves bleeding hot air off from the engines,and the hot air is then ducted to the airframe components such as theleading edges of the wing and the tail which are prone to ice formation.More recently, electrically powered systems have been proposed, such asin EP-A-1,757,519 (GKN Aerospace) which discloses a wing slat having anose skin which incorporates an electrothermal heater blanket or mat.The heater mat is bonded to the rear surface of a metallic erosionshield which comprises the forwardly-facing external surface of the noseskin.

The heater mat is of the “Spraymat” (trade mark) type and is a laminatedproduct comprising dielectric layers made of preimpregnated glass fibrecloth and a heater element formed by flame spraying a metal layer ontoone of the dielectric layers. The “Spraymat” has a long history from itsoriginal development in the 1950s by D. Napier & Sons Limited (see theirGB-833,675 relating to electrical de-icing or anti-icing apparatus foran aircraft) through to its subsequent use by GKN Aerospace.

A recent “Spraymat” produced by GKN Aerospace for use in a wing slat isformed on a male tool and involves laying up a stack of plies comprising(i) about 10 layers of glass fibre fabric preimpregnated with epoxycured in an autoclave, (ii) a conductive metal layer (the heaterelement) which has been flame sprayed onto the laminate using a mask toform the heater element pattern and (iii) a final 3 or so layers of theglass fibre fabric. Wiring is soldered to the heater element to permitconnection to the aircraft's power system. The heater mat is then curedin an autoclave.

A heater mat usually incorporates a temperature sensor as part of acontrol loop to provide temperature control andthermal-damage-prevention information to a control unit which isconnected to the heater mat. The temperature sensor is usually aresistance temperature device (RTD) sensor with a sensing headencapsulated within a polyimide film such as Kapton (trade mark).Polyimide is a thermoplastic material with an alternative usage as arelease or parting film within laminates. This characteristic isundesirable when polyimide is being used in a temperature sensor whichis to be incorporated within a laminated heater mat as the temperaturesensor will provide a discontinuity within the heater mat. Thediscontinuity will act as a crack initiation site within the laminatedheater mat, thereby providing a site of potential structural weakness orfatigue weakness.

It would be desirable to provide an improved heater mat.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is providedan electrothermal heater mat for an ice protection system, wherein:

-   -   the heater mat is a laminated heater mat and comprises        dielectric layers, a heater element and a temperature sensor;    -   each dielectric layer comprises thermoplastic material;    -   the temperature sensor comprises a sprayed metal track deposited        on a substrate comprising thermoplastic material;    -   the substrate is laminated to at least a first one of the        dielectric layers; and    -   the thermoplastic material of the substrate is (i) the same as        the thermoplastic material of the first dielectric layer such        that the thermoplastic material of the substrate is dispersed or        merged into the thermoplastic material of the first dielectric        layer or (ii) compatible with the thermoplastic material of the        first dielectric layer such that the thermoplastic material of        the substrate is fused to the thermoplastic material of the        first dielectric layer.

Because the thermoplastic material of the substrate is the same as or iscompatible with the thermoplastic material of the first dielectric layerto which it is laminated, the formation of an undesirable discontinuityat the interface between the substrate and the first dielectric layer issubstantially prevented or minimised. Thus, cracks are less likely to beinitiated at the interface during the use of the heater mat, andde-lamination is less likely to occur. In other words, the structural orfatigue strength is improved.

If the same thermoplastic material is used for the substrate and all ofthe dielectric layers, the lamination can be performed such that thereare substantially no discontinuities between any of the thermoplasticcomponents of the heater mat. This gives the thermoplastic of the heatermat a monolithic structure which will resist de-lamination during use ofthe heater mat.

If the thermoplastic material of the substrate is not the same as thatof the first dielectric layer and is merely compatible with the materialof the first dielectric layer, then the compatibility can be achieved byselecting the thermoplastic of the substrate such that it is notnecessary to use adhesive to bond it to the thermoplastic of the firstdielectric layer during the lamination. The dissimilar but compatiblematerials will bond to one another at the interface by one thermoplasticmaterial (e.g. PEEK) fusing to but not dispersing into the otherthermoplastic material (e.g. PEKK) when the stack of assembledcomponents is heated to above the melt point of one of the abuttingthermoplastic materials.

In some of our current embodiments, the substrate is a second one of thedielectric layers. Thus, a dielectric layer that is in any event goingto be provided may also be used for the purpose of receiving the sprayedmetal track of the temperature sensor.

Preferably, the sprayed metal track of the temperature sensor is porousand the thermoplastic material of the first dielectric layer islaminated to the thermoplastic material of the second dielectric layerthrough the sprayed metal track of the temperature sensor. This furtherimproves the strength of the lamination between the first and seconddielectric layers.

Conveniently, the heater element comprises a sprayed metal trackdeposited on the second dielectric layer. Thus both the temperaturesensor and the heater element are present on the same substrate. Asingle dielectric layer may be efficiently processed to receive both thetemperature sensor and the heater element by using a single flamespraying machine to spray the metals of the various tracks. Because thetemperature sensor and the heater element are positioned on differentzones of the dielectric layer, their metal tracks could be sprayedsimultaneously instead of sequentially by using respective masks whichdefine the shapes of the metal tracks.

In an alternative embodiment, the substrate is a carrier which has amain surface which is smaller than a main surface of a second one of thedielectric layers onto which the carrier is laminated, and the carrieris sandwiched between the first and second dielectric layers. Thus, thetemperature sensors on their carriers may be pre-manufactured in batchesbefore the main manufacturing of the heater mat, and when the heater matis being manufactured a desired quantity (e.g. one, or more than one) oftemperature sensors on their carriers may be included in the stack ofcomponents that are to be laminated together to form the heater mat. Thenumber and positioning of the temperature sensors can be selected by thedesigner of the heater mat.

The or each carrier may cover 10% or less of said main surface of thesecond dielectric layer, or 5% or less, or 2% or less.

Preferably, the sprayed metal track of the temperature sensor is porousand the thermoplastic material of the first dielectric layer islaminated to the thermoplastic material of the carrier through thesprayed metal track of the temperature sensor.

In our current embodiments, the sprayed metal track of the temperaturesensor comprises a sensor head. For example, the metal of the sensorhead may be nickel or nickel alloy. Also, the sensor head extendsbetween intermediate positions of the sprayed metal track of thetemperature sensor, and the sprayed metal track of the temperaturesensor further comprises leads which extend from the intermediatepositions to terminals at the ends of the sprayed metal track. Forexample, the metal of the leads may be copper or copper alloy.

An encapsulation layer may comprise the same thermoplastic material asthe thermoplastic material of the carrier, and the encapsulation layeris laminated to the carrier and covers part of the sprayed metal trackof the temperature sensor. For example, the encapsulation layer maycover the sensor head and part of each lead.

In our current embodiments, we use high-temperature engineeringthermoplastic. Our preferred material comprises PEEK, PEKK, PPS, PEI orPES or a mixture thereof. These materials are able to withstand flamespraying of the sprayed metal track(s) without significant damage. Weparticularly prefer PEEK and PEKK.

Preferably, the substrate and the dielectric layers all comprise thesame thermoplastic material. This optimises the strength of thelamination of the components when the stack of assembled components isheated up and pressed together to form the laminated heater mat.

An electrothermal ice protection system may comprise an electrothermalheater mat in accordance with the present invention, and a connectorhaving a first end which is preferably embedded in the heater mat and iselectrically connected to the heater element of the heater mat and asecond end which extends away from the heater mat and is connected to aheater control unit.

Ice protected apparatus may comprise an external skin and anelectrothermal heater mat which is in accordance with the presentinvention and is in thermal contact with a rear surface of the externalskin.

According to a second aspect of the present invention, there is provideda method of manufacturing an electrothermal heater mat, comprising thesteps of:

-   -   providing a plurality of dielectric layers each comprising        thermoplastic material;    -   flame spraying a metal track of a temperature sensor onto        thermoplastic material of a substrate;    -   forming a stack comprising the dielectric layers, a heater        element and the substrate; and    -   laminating together the dielectric layers and the substrate such        that the thermoplastic material of the substrate (i) disperses        or merges into or (ii) is fused to the thermoplastic material of        the or each adjacent one of the dielectric layers.

In some of our current embodiments, the substrate is a first one of thedielectric layers and the method further comprises the step of flamespraying the heater element on said first dielectric layer.

The flame sprayed temperature sensor and heater element are porous, andduring the lamination the adjacent thermoplastic material migrates intoor through the pores to reduce or eliminate any discontinuity at thetemperature sensor and heater element. This migration of thethermoplastic material reduces the risk of de-lamination occurring atthe temperature sensor and the heater element.

In an alternative embodiment, the substrate is a carrier which has amain surface which is smaller than a main surface of a first one of thedielectric layers onto which the carrier is laminated during thelaminating step, and the carrier is sandwiched between said firstdielectric layer and a second one of the dielectric layers during thestack forming step.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings inwhich:—

FIG. 1 is a diagrammatic plan view of an aircraft having slats in theleading edge of a wing.

FIG. 2 is a diagrammatic perspective view of a nose skin of a wing slatof FIG. 1.

FIG. 3 is a diagrammatic perspective view of a dielectric layer at afirst stage of an assembly process for producing a heater mat inaccordance with a first embodiment of the present invention.

FIG. 4 is a diagrammatic perspective view of the dielectric layer ofFIG. 3 at a second stage of the assembly process.

FIG. 5 is a diagrammatic perspective view of the dielectric layer ofFIG. 4 at a third stage of the assembly process.

FIG. 6 is a diagrammatic cross-sectional view taken on the section linein FIG. 5.

FIG. 7 is a diagrammatic enlargement of the circled area of FIG. 6.

FIG. 8 is a diagrammatic perspective view of two connectors for use inthe assembly process.

FIG. 9 is a diagrammatic perspective view of the dielectric layer ofFIG. 5 at a fourth stage of the assembly process when being assembledwith connectors of the type shown in FIG. 8.

FIG. 10 is a diagrammatic perspective view of the partially-assembledheater mat of FIG. 9 at a fifth stage of the assembly process afterbeing assembled with a further dielectric layer.

FIG. 11 is a diagrammatic perspective view of the partially-assembledheater mat of FIG. 10 at a sixth stage of the assembly process afterbeing flame sprayed with a copper ground plane.

FIG. 12 is a diagrammatic cross-sectional enlargement of the circledarea of FIG. 11 and shows the interface between the ground plane and thedielectric layer on which the ground plane has been sprayed.

FIG. 13 is a diagrammatic perspective view of the partially-assembledheater mat of FIG. 11 at a seventh stage of the assembly process after aconnector of the type shown in FIG. 8 has been connected to the groundplane.

FIG. 14 is a diagrammatic perspective view of the partially-assembledheater mat of FIG. 13 at an eighth stage of the assembly process after afurther dielectric layer has been added.

FIG. 15 is a diagrammatic perspective view of the partially-assembledheater mat of FIG. 14 at a ninth stage of the assembly process after afurther dielectric layer has been added.

FIG. 16 is a diagrammatic perspective view of the partially-assembledheater mat of FIG. 15 at a tenth stage of the assembly process after theflame spraying of a second ground plane.

FIG. 17 is a diagrammatic perspective view of the partially-assembledheater mat of FIG. 16 at an eleventh stage of the assembly process aftera further dielectric layer and a connector of the type shown in FIG. 8have been added.

FIG. 18 is a diagrammatic perspective view of the heater mat of FIG. 17at a twelfth stage of the assembly process after the assembledcomponents of the heater mat have been laminated together.

FIG. 19 is a diagrammatic perspective view showing the heater mat ofFIG. 18 being bonded to an erosion shield.

FIG. 20 is a diagrammatic perspective view of an intermediate stage ofan alternative assembly process for producing a heater mat in accordancewith a second embodiment of the present invention.

FIG. 21 is a diagrammatic perspective view of the partially-assembledheater mat of FIG. 20 at a subsequent stage of the alternative assemblyprocess.

FIG. 22 is a diagrammatic perspective view of the partially-assembledheater mat of FIG. 21 at a subsequent stage of the alternative assemblyprocess.

FIG. 23 is a diagrammatic perspective view of the partially-assembledheater mat of FIG. 22 at a subsequent stage of the alternative assemblyprocess.

FIG. 24 is a diagrammatic perspective view of the partially-assembledheater mat of FIG. 23 at a subsequent stage of the alternative assemblyprocess.

FIG. 25 is a diagrammatic perspective view of the partially-assembledheater mat of FIG. 24 at a subsequent stage of the alternative assemblyprocess.

FIG. 26 is a diagrammatic perspective view of the partially-assembledheater mat of FIG. 25 at a subsequent stage of the alternative assemblyprocess.

FIG. 27 is a diagrammatic perspective view of the heater mat of FIG. 26after the components thereof have been laminated together.

FIG. 28 is a diagrammatic perspective view showing the heater mat ofFIG. 27 of the second embodiment of the present invention when beingassembled to an erosion shield.

FIG. 29 is a diagrammatic perspective view of an alternative areatemperature sensor.

FIG. 30 is a diagrammatic perspective view of the area temperaturesensor of FIG. 29 after being assembled onto the dielectric layer ofFIG. 3.

FIG. 31 is a diagrammatic perspective view of an alternative connector.

FIG. 32 is a diagrammatic perspective view of a further alternativeconnector.

FIG. 33 is a schematic view showing the connections between a heater matin accordance with the present invention and a power supply and controlelectronics unit of an aircraft.

While the invention is susceptible to various modifications andalternative forms, specific embodiments are shown by way of example inthe drawings and are herein described in detail. It should beunderstood, however, that the drawings and detailed description of thespecific embodiments are not intended to limit the invention to theparticular forms disclosed. On the contrary, the invention is cover allmodifications, equivalents and alternatives falling within the spiritand the scope of the present invention as defined by the appendedclaims.

DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 is a plan view of an aircraft 1 having a wing 11 along theleading (forward) edge of which are positioned five wing slats 12. Eachwing slat 12 incorporates an electrothermal ice protection system.

FIG. 2 is a diagrammatic perspective view of a demountable nose skin 13of one of the wing slats 12 of FIG. 1. The configuration of the noseskin 13 may be generally the same as in EP-A-1,757,519 (GKN Aerospace)which discloses a wing slat having a demountable forward sectioncomprising a nose skin.

The nose skin 13 comprises an erosion shield 14 and anelectrically-powered heater 2.

The heater 2 comprises a heater blanket or mat 3 and a bundle ofconnectors 4 which connect the heater mat 3 to the power supply andcontrol electronics of the aircraft 1.

The erosion shield 14 is generally rectangular and has a front surface141 which is convexly curved and a rear surface 142 which is concavelycurved. An apex 1411 of the front surface 141 provides the leading edgeof the aircraft wing 11.

The heater mat 3 is generally rectangular and has a front surface 31which is convexly curved and a rear surface 32 which is concavelycurved. The convex front surface 31 conforms to the shape of and isbonded to the rear surface 142 of the erosion shield 14. In this way,thermal energy generated as the heater mat 3 is operated passes, byconduction, into the erosion shield 14 in order to provide an iceprotection function. The erosion shield 14 is metallic and may be madeof aluminium (which is the usual material) or titanium (which isexpensive but may offer some functional and processing benefits). Animportant function of the erosion shield 14 is to protect the aircraftagainst lightning strikes by absorbing and dissipating the lightningcurrent.

The demountable nose skin 13 is convenient because just the nose skinmay be removed from the main or rear section of the wing slat 12 toenable the nose skin to be repaired or replaced if it has been damaged,or to enable maintenance to be performed on the heater 2.

If the heater 2 has developed a fault, the nose skin 13 may be demountedfrom the main or rear section of the wing slat 12 by, for example,undoing or releasing releasable securing means such as screws. Theheater 2 may then be inspected and tested. If possible, the heater 2 isrepaired in situ. If this is not possible, the heater mat 3 is removedfrom the erosion shield 14 of the nose skin 13 and a heater mat of a newheater is secured to (e.g. bonded or glued onto) the erosion shield 14.The nose skin 13 is then ready to be returned to service.

Whilst the old nose skin is being repaired, a new nose skin taken fromstock may be fitted to the wing slat 12 to keep the aircraft in flyingcondition.

An assembly process for producing a heater mat in accordance with thefirst embodiment of the present invention will now be described withreference to FIGS. 3-19 which depict, in a very diagrammatic manner, thecomponents of the heater mat and how they are assembled together toproduce the heater mat and how the heater mat is then bonded to anerosion shield.

The components shown in FIGS. 3-19 are very diagrammatic. For example,in relation to the dielectric layer 50 shown in FIG. 3, the thicknesshas been exaggerated for the sake of clarity. Also, the width and lengthof the layer have been reduced for the sake of clarity. In a practicalembodiment, the dielectric layer would be generally rectangular andwould be a sheet having a width ranging typically from 0.25 m to 1 m anda length ranging from typically 1 m to 4 m. In use, the width of thesheet will usually wrap around the chord at the leading edge of thewing, and the length of the sheet will usually extend along the span ofthe wing. The dielectric sheet (the dielectric layer) would alsotypically have a thickness of 0.05 mm to 2 mm.

The dielectric layer 50 is made from a high-temperature engineeringthermoplastic or from a reinforcement material (such as glass fibres)which is impregnated with the high-temperature engineeringthermoplastic.

From the class of high-temperature engineering thermoplastics, wecurrently use: PEEK (polyether ether ketone), PEKK(polyetherketoneketone), PPS (polyphenylene sulphide), PEI(polyetherimide) or PES (polyethersulphone) or mixtures thereof. Thesematerials have been selected based on the requirement for a suitableglass transition temperature and suitable thermal fatigue performance.PEEK and PEKK are particularly preferred because PEEK has the necessarymechanical performance and is particularly receptive to a flame sprayedmetal coating, and PEKK has similar properties but is easier to bond tothe metal material.

The other components of the heater mat (to be described later) are eachselected to be made from a material the same as or compatible with thematerial of the dielectric layer 50 so that, when the components arelaminated together at the end of the assembly process, the componentscan merge or fuse together so that the heater mat is monolithic. Thismeans that the laminated components of the heater mat will notdelaminate from one another. Because of the absence of discontinuitiesbetween discrete layers, it is not possible for cracks to initiate atthe (former) boundaries between adjacent substrate layers, and thisimproves the fatigue resistance of the heater mat.

FIG. 3 shows that the track of a heater element 501 has been laid downon the upper main surface 502 of the dielectric substrate layer 50. Theheater element 501 extends from a first terminal 503 to a secondterminal 504. The heater element 501 is shown in FIG. 3 as having asimple “C” shape. In practice, it will have a more complicated shapesuch as a shape that repeatedly zigzags from the first terminal 503 tothe second terminal 504. The heater element 501 is shown in FIG. 3 ashaving a simple shape for the sake of clarity of the diagrammaticdepiction.

The dielectric layer 50 has four through holes 505 which extend from theupper main surface 502 through to a lower main surface 506 (see FIG. 5).A mask is used to flame spray the track of the heater element 501 ontothe upper main surface 502 of the dielectric layer 50 so that the trackruns from the first terminal 503 to the second terminal 504. The heaterelement 501 is made of a resistive metal such as copper or metal alloysuch as a copper-manganese alloy. Flame or hot metal spraying is awell-established technique dating back many years, for example back toGB-833,675 (D. Napier & Sons Limited) which relates to hot metalspraying various metal layers of an electrical de-icing or anti-icingapparatus for an aircraft, and the reader is referred to GB-833,675which is incorporated herein by reference. The spraying is performed sothat the heater element 501 is porous, with the degree of porositydepending on the number of passes of the spraying gun and the thicknessof the metal coating that forms the heater element 501. A suitablespraying gun is the Mark 66E-Man produced by Metallisation Limited ofDudley, West Midlands, United Kingdom in combination with its associatedcontrol equipment.

The through holes 505 are formed before the flame spraying of the heaterelement 501. Each hole has a typical diameter of 3.5 mm, but may rangefrom 1 to 6 mm in diameter, more preferably 2 to 5 mm in diameter, or 3to 4 mm in diameter. During the flame spraying, some of the material ofthe heater element 501 is sprayed down into the two holes 505 at thefirst and second terminals 503, 504.

The next stage of the assembly process is shown in FIG. 4. A mask isused to flame spray an area temperature sensor 507 onto the upper mainsurface 502. Thus, the temperature sensor 507 is present on the samesubstrate layer as the heater element 501. The track of the temperaturesensor 507 extends from a first terminal 508 to a second terminal 509.Each of the terminals 508, 509 is located at a respective through hole505. During the flame spraying, some of the material of the temperaturesensor 507 is sprayed down into the two holes 505 at the terminals 508,509.

The area temperature sensor 507 is used as part of a control loop toprovide temperature control and thermal-damage-prevention information toa control unit for the heater 2. The temperature sensor 507 is aresistance temperature device (RTD) sensor. The flame spraying lays downa conductive metal track having a suitable temperature coefficient ofresistance. Suitable metals include nickel and nickel-based alloys,although any metal with a high temperature coefficient of resistancecould be used as long as it is suited to being applied by a flamespraying process. The conductive metal coating may be used to form theentirety of the temperature sensor 507 from the first terminal 508 tothe second terminal 509. Alternatively, as shown in FIG. 4, theconductive metal coating with the suitable temperature coefficient ofresistance may be flamed sprayed to form a sensor head 5010 locatedbetween two intermediate boundaries 5011 on the track of the sensor 507.Leads 5012 may be flamed sprayed from the boundaries 5011 to theterminals 508, 509 so as to connect the sensor head 5010 to theterminals 508, 509. The leads 5012 may be a conductive metal such ascopper.

The next stage of the assembly process is shown in FIG. 5. It involvesturning over the dielectric layer 50 so that the lower main surface 506is facing upwards. Then, a mask is used to spray conductive metal (e.g.copper) or alloy to form terminals or contact pads 5013 around thethrough holes 505. During this flame spraying, some of the material ofthe terminal 5013 coats the bore of each through hole 505. This is moreclearly shown in FIG. 6 which is a cross-sectional view taken on thesection line in FIG. 5. The heater element 501 is shown in FIG. 6 ashaving a generally cylindrical projection 5014 which extends into thethrough hole 505 from the main surface 502 and forms a radially outercoating inside the through hole 505.

The terminal 5013 is shown as having a generally cylindrical projection5015 which extends into the hole 505 from the main surface 506 and formsa radially inner coating of the through hole 505.

In FIG. 6, the cylindrical projection 5015 of coating material is shownas leaving the hole 505 as having a through bore 5016. If the coatingthickness of the terminal 5013 and its projection 5015 is sufficientlythick, and/or if the diameter of the through hole 505 is sufficientlysmall, it is possible that the projection 5015 will occlude or block theradially inner part of the through hole 505 so as to form a centralplug. Under these circumstances, there would be no through bore 5016after the two coatings 501, 5013 have been applied.

As shown in FIG. 6, the free end 5017 of the coating projection 5014extends beyond the free end 5018 of the coating projection 5015. Thus,the projection 5014 overlaps the projection 5015 within the hole 505.The free end 5017 is shown as stopping short of the main surface 506,but it could extend substantially to the main surface 506 and evensometimes extend slightly onto the main surface 506. This might occur,for example, if the sheet of dielectric material forming the layer 50 ispositioned on the table of a metal spraying machine and the sheetvibrates during the flame spraying. This vibration would facilitate a“through plating” effect where the sprayed metal passes all the waythrough the hole 505 and carries on slightly to coat the far surface 506around the hole 505.

Similarly, the coating projection 5015 of the terminal 5013 is shown ashaving its free end 5018 stopping short of the main surface 502. Theflame spraying or other application process could be arranged to ensurethat the free end 5018 extends substantially to the main surface 502 or,perhaps, even extends round onto part of the main surface 502 adjacentto the through hole 505. Of course, under these circumstances, theheater element 501 would be interposed between the free end 5018 and themain surface 502.

Because of the overlap between the free end 5017 and the free end 5018,there is a continuous conductive path between the main surface 502 andthe main surface 506. This is true of each of the through holes 505which is subjected to the “spray plating” from both ends to form acontinuous through connection.

In order to achieve a satisfactory through connection, it is beneficialfor the dielectric layer to have a thickness in the range of 0.05 mm to2 mm.

FIG. 7 is a diagrammatic enlargement of the circled area in FIG. 6 andshows the overlap between the two coatings forming the projections 5014,5015. The flame spraying produces a coating having particles with a meandiameter typically between 30-150 μm. Also, each coating 5014, 5015forms a microporous conductor. The particles of the coatings at theinterface between the projection 5014 and the projection 5015 are inintimate contact in order to form a good electrical connection betweenthe heater element 501 and the terminal 5013.

FIG. 8 shows two connectors 41, 42 which comprise part of the bundle ofconnectors 4 shown in FIG. 2 and which are used to electrically connectthe heater mat 3 to the power supply and control electronics unit 6 (seeFIG. 33) of the aircraft 1.

Each of the connectors 41, 42 comprises a dielectric substrate layer411, 421 which is a strip having the desired length for the connector toperform its connection function.

Each substrate layer 411, 421 is made of high-temperature engineeringthermoplastic which is the same as or compatible with the materials ofthe other component dielectric layers and connectors of the heater 2 sothat, when at the end of the assembly process the components of theheater are laminated together, the substrate layers 411, 421 willsatisfactorily disperse into the adjacent dielectric layer(s) and/orconnector(s) so that the components of the heater form a satisfactorymonolithic unit without having to use glue to connect the dielectricsubstrate layers and connectors together.

Thus, the currently preferred materials for the dielectric substratelayer 411 or 421 are PPS, PEI, PEKK, PEEK and PES. Of these materials,we currently particularly prefer PEKK or PEEK. These materials areparticularly good at ensuring that the components of the heater 2 willfuse or bond together to become monolithic and will not delaminate.

Preferably, each substrate layer 411, 421 is made of the samethermoplastic material as the other components as this helps to ensurethat the stack of assembled components will disperse or merge into oneanother to form the monolithic unit when the thermoplastic material isheated to above its melt point and pressure is applied to the stack.

If the material of each substrate layer 411, 421 is not the same as thatof the other components and is merely compatible with the material ofthe other components, then the compatibility can be achieved byselecting the thermoplastic of the substrate layers 411, 421 such thatit is not necessary to use adhesive to bond it to the thermoplastic ofthe other components in the stack during the lamination. The dissimilarbut compatible materials will bond to one another at each interface byone thermoplastic material (e.g. PEEK) fusing to but not dispersing intothe other thermoplastic material (e.g. PEKK) when the stack of assembledcomponents is heated to above the melt point of one of the abuttingmaterials.

After a sheet of dielectric material has been cut to form theribbon-like substrate layers 411, 421 a mask is then used to flame spraya conductive metal (e.g. copper) or metal alloy onto a main surface 412,422 so as to lay down power or signal tracks. In the case of theconnector 41, a power track 413 is laid down in the longitudinaldirection of the dielectric strip 411 and terminates in a terminal 414at an end 415 of the connector 41.

In the case of the connector 42, flame spraying is used to lay down thetwo generally-parallel signal tracks 423 each of which terminates at aterminal 424 at an end 425 of the connector 42.

The other end of each of the tracks 413, 423 may be terminated in anysuitable manner for connection to the power supply and controlelectronics unit 6.

FIG. 9 shows the next stage of the assembly process. In this stage, thedielectric layer 50 of FIG. 5 is assembled with two connectors 41 and asingle connector 42. The three connectors 41, 42 are positioned, asshown in FIG. 9, with their terminals 414, 424 facing downwards towardsthe terminals 5013 of the dielectric layer 50. The terminals 414, 424are then welded or soldered to the terminals 5013.

In this way, the two connectors 41 are connected to the ends of theheater element 501 so that the heater element 501 can be powered by thepower supply and control electronics unit 6 via the connectors 41. Theends of the temperature sensor 507 are connected via the connector 42 tothe power supply and control electronics unit 6.

FIG. 10 shows the next stage of the assembly process. Thepartially-assembled heater mat of FIG. 9 has a further dielectric layer51 positioned on the main surface 506 of the dielectric layer 50. Thedielectric layers 50, 51 are made of the same material, such as PEEK orPEKK.

In FIG. 10, the dielectric layer 51 does not cover the ends 415, 425 ofthe connectors 41, 42 but it could be arranged to cover the ends so thatthe dielectric layer 51 is generally the same size and shape as thedielectric layer 50. During lamination at the end of the assemblyprocess, the increased thickness of dielectric material at the ends 415,425 will be, at least partially, dispersed or spread out as a result ofthe heat and pressure applied during lamination. Furthermore, in thefinished heater mat 3, it does not matter if, to some extent, the endproduct (the laminated product) is locally slightly thicker in places asa result of an increased thickness of dielectric material being present.

The next stage of the assembly process is shown in FIG. 11. In thisstage, a ground plane 71 is flame sprayed onto the upper main surface511 of the dielectric layer 51 of the partially-assembled heater mat ofFIG. 10. The ground plane comprises flame sprayed copper or copper alloyand is typically 0.05 mm thick, but may range from 0.01 mm to 0.5 mm inthickness, or from 0.03 mm to 0.2 mm in thickness. The exact thicknesscan be chosen depending on the conductivity that is required.

The purpose of the ground plane 71 is to detect a fault current causedby a heater fault in the heater element 501. For example, the faultcould be damage such as heater burn-out. The ground plane 71 isconnected to the aircraft earth 143 (see FIG. 19) as well as to thepower supply and control electronics unit 6, so that when a fault occursthe unit 6 detects a change in current.

FIG. 12 is a diagrammatic cross-sectional enlargement of the circledarea of FIG. 11 and shows the interface between the ground plane 71 andthe dielectric layer 51 onto which the ground plane has been sprayed.The particles of the ground plane 71 are micro-porous so that, duringthe heating and pressing of the lamination process, the thermoplastic ofthe adjacent dielectric layers will pass or migrate through the groundplane 71 as part of giving a monolithic structure to the heater mat 3.This migration is indicated by the arrows 711 which show migration pathsbetween the particles 712 of the ground plane 71. Note that, in FIG. 12,only some of the particles 712 are labelled for clarity. The particles712 are randomly positioned as a result of the spraying and have arandom range of sizes with the mean diameter, typically ranging from30-150 μm.

The next stage of the assembly process is shown in FIG. 13.

In this stage, a connector 43, which is the same as connector 41, iselectrically connected to the ground plane 71 of the partially-assembledheater mat of FIG. 11. The connector 43 has a track 433 on its bottomsurface which terminates at a terminal, and that terminal is welded orsoldered to the ground plane 71. In this way, the ground plane 71 iselectrically connected via the connector 43 to the power supply andcontrol electronics unit 6.

The next stage of the assembly process is shown in FIG. 14. A dielectriclayer 52 is laid on top of the ground plane 71 of thepartially-assembled heater mat of FIG. 13. The dielectric layer 52 ismade of the same material as the dielectric layers 50, 51. It is shownas having a cutout in the region of the connector 43. However, thedielectric layer 52 could be the same size and shape as the dielectriclayer 50 such that it would cover the end 435 of the connector 43.

The next stage of the assembly process is shown in FIG. 15. Thepartially-assembled heater mat of FIG. 14 is turned upside down and afurther dielectric layer 53 is positioned on the main surface 502 of thedielectric layer 50. The dielectric layer 53 is the same size and shapeas the dielectric layer 50 and it is made of the same material as theother dielectric layers 50, 51 and 52. In FIG. 15, it is possible to seethe tracks 413 of the connectors 41, the tracks 423 of the connector 42,and the track 433 of the connector 43.

The next stage of the assembly process is shown in FIG. 16. In thisstage, the partially-assembled heater mat of FIG. 15 has a second groundplane 72 flame sprayed onto the exposed main surface 531 of thedielectric layer 53. The characteristics of the second ground plane 72are the same as those of the first ground plane 71. In particular, it ispreferable that the ground planes 71, 72 should be flame sprayed copper.

The next stage of the assembly process is shown in FIG. 17. In thisstage, a further dielectric layer 54 is positioned on top of the groundplane 72 of the partially-assembled heater mat of FIG. 16. Thedielectric layer 54 is made of the same material as the other dielectriclayers 50, 51, 52, 53. A connector 44 is generally the same as theconnector 41 and has, on it undersurface in FIG. 17, a track leading toa terminal at the end 445 of the connector 44. This terminal of theconnector 44 is electrically connected to the second ground plane 72 bywelding or soldering so as to establish an electrical connection betweenthe ground plane 72 and the power supply and control electronics unit 6.

Collectively, the connectors 41, 42, 43, 44 comprise the bundle ofconnectors 4 which is diagrammatically shown in FIG. 2.

In FIG. 17, the dielectric layer 54 is shown as having a cutout aroundthe end 445 of the connector 44. An alternative would be for the layer54 to omit the cutout, such that the layer 54 has the same rectangularshape and size as the underlying dielectric layer 53. This would meanthat the dielectric layer 54 would cover the end 445 of the connector44. This might result, after lamination, in a slight local increase inthickness of the heater mat in the vicinity of the end 445.

During the laying up of the dielectric layers, reinforcement materialmay be included in the stack of components of the heater mat. Thereinforcement material would be fibrous and examples of thereinforcement material include glass fibres, e.g. either as auni-directional tape or as a woven fabric, which would be porous to theadjacent thermoplastic dielectric layers during the lamination process.Any reinforcement would need to be non-conductive in order to preservethe insulation provided by the dielectric layers. Also, thereinforcement material should be selected to be as thin as possible.

In FIG. 17, all of the components of the heater mat 3 are in positionready to be laminated together. The lamination process isdiagrammatically illustrated in FIG. 18. Heat and pressure are appliedto the stack of components so as to consolidate the laminate into amonolithic structure. The result is that the dielectric layers and theembedded ends of the connectors, all being made of the same orcompatible engineering thermoplastics, disperse into one another, andthe dielectric layers and the ends of the connectors merge or fusetogether to become monolithic. Consequently, the layers and the ends ofthe connectors will not delaminate as a result of the presence of adiscontinuity at an interface caused by thermoplastic material which isincompatible and has not merged with the adjacent thermoplasticmaterial. During the lamination, the embedded ends of the connectorseffectively become part of the heater mat.

Lamination may be performed using a conventional autoclave, heated pressor large laminating machine. Such machinery can be used to heat thestack of components to above the melt point of the thermoplasticmaterial whilst applying pressure, in order to consolidate the laminate.

If reinforcement material is present in the stack of components, thepressure of the lamination process presses the reinforcement materialinto the thermoplastic of the adjacent layers to form a reinforcedthermoplastic laminate. If the reinforcement material is a woven fabric,care should be taken to ensure that the treatments applied to it duringthe weaving and finishing processes are compatible with laminationtemperatures in the order of 400° C.

The intention of the lamination process is to minimise or eliminatediscontinuities in the resulting laminate. The end product in the formof the heater mat 3 with the embedded ends of the bundle of connectors 4has a monolithic structure which can undergo generally uniform expansionas it is heated up. This reduces the thermomechanical stresses on theheater mat 3. This is an important consideration in view of the factthat the thermomechanical stresses are greater than the aerodynamicstresses that the heater mat 3 experiences when installed in theaircraft 1.

In conventional laminated products, glue is used and glue is a weakpoint at the interfaces between adjacent layers of the laminate. In aconventional heater where the dielectric layers are glued together inthe laminate, the glued interfaces are where delamination can occurunder fatigue loadings.

An advantage of the heater mat of the first embodiment of the presentinvention as shown in FIG. 18 is that it is glue free. Specifically,glue is not used to laminate together the dielectric layers and theembedded ends of the connectors.

FIG. 19 shows how the heater mat 3 is offered up to the rear surface 142of the erosion shield 14. A suitable adhesive is used to glue or bondthe front surface 31 of the heater mat 3 to the rear surface 142 of theerosion shield 14. For ease of illustration, in FIG. 19 the heater mat 3and the erosion shield 14 are shown as being planar. In an actualrepresentative installation such as shown in FIG. 2, the front surface31 is convexly curved and the rear surface 142 is correspondinglyconcavely curved. The heater mat 3 resembles a large sheet which iscomparatively long and wide relative to its thickness, and thus theheater mat 3 is flexible and may be easily bent to conform to the shapeof the rear surface 142 of the erosion shield 14.

When the heater mat 3 has been installed behind the erosion shield 14,and when the nose skin 13 is being fitted onto the aircraft 1, theconnectors 41, 42, 43 and 44 (which collectively form the bundle ofconnectors 4) may be connected to the power supply and controlelectronics unit 6 of the aircraft 1. Thus, the heater 2 is now readyfor use.

In the first embodiment of the heater mat as discussed above withreference to FIGS. 3-19, it is the case that the heater mat incorporatestwo ground planes (ground plane 71 and ground plane 72). When theaircraft 1 is struck by lightning on the erosion shield 14, a very largedirect current (e.g. 200,000 amps) of a very short duration isdissipated to an aircraft earth 143 by the erosion shield 14. The verylarge current flowing in the erosion shield during the lightning strikewill induce a current in any underlying parallel conductor as a resultof electromagnetic coupling. Such parallel conductors include the heaterelement 501 and the temperature sensor 507. If the heater element 501and the temperature sensor 507 are not adequately shielded from theelectromagnetic coupling, the current that is induced in them may be ofthe order of 1,000 amps and this current might pass along the bundle ofconnectors 4 to the power supply and control electronics unit 6. Theresult could be a current surge in the power supply and controlelectronics unit 6, which is only designed to cope with currents in theorder of 10 amps. A current surge is undesirable as it might damage theelectronics inside the unit 6.

In relation to a conventional heater mat with a single ground plane,some current will be induced in the ground plane and will pass to theaircraft earth.

In the heater mat 3 of the first embodiment of the present invention, asdisclosed with referenced to FIGS. 3-19, it is the case that the heatermat 3 incorporates two ground planes 71, 72. These ground planes 71, 72are positioned above and below the heater element 501 and thetemperature sensor 507 so that the heater element 501 and temperaturesensor 507 are “electromagnetically shielded” by the two ground planes71, 72. This shielding is rather similar to the concept of coaxialshielding in a cable.

The ground planes generally have a low resistance. Because the twoground planes sandwich the vulnerable heater element 501, thetemperature sensor 507 and the embedded ends of the connection bundle 4which are connected to the heater element 501 and the temperature sensor507, they shield those components and the induced current during alightning strike is preferentially induced in the two ground planes 71,72 and passes to the aircraft earth 143. Much-reduced currents areinduced in the heater element 501, the temperature sensor 507 and theembedded ends of the connection bundle which lead away from the heaterelement 501 and the temperature sensor 507, thereby reducing the risk ofdamage to the electronics in the power supply and control electronicsunit 6.

There will now be described an alternative build process. Specifically,FIGS. 20-28 illustrate the relevant aspects of an alternative assemblyprocess for producing a heater mat in accordance with the secondembodiment of the present invention. FIGS. 20-28 illustrate only thoseaspects of the build process that differ from what is shown in FIGS.3-19 in relation to the first embodiment of the present invention.

Thus, in FIG. 20, the second embodiment takes the dielectric layer 50 ofFIG. 4 of the first embodiment and turns it upside down, and then aground plane 73 is flamed sprayed onto the main surface 506 of thedielectric layer 50 such that the ground plane 73 has the samecharacteristics as the ground plane 71.

Then, in the next stage of this alternative assembly process of thesecond embodiment, a dielectric layer 55 is positioned on top of theground plane 73 (see FIG. 21). The dielectric layer 55 is made of thesame material as the dielectric layer 50. A connector 45 (whichcorresponds to the connector 43 of the first embodiment) is electricallyconnected to the ground plane 73. The dielectric layer 55 has a cutoutaround the end 455 of the connector 45, but this cutout may be omittedand the dielectric layer 55 may have the same size and shape as thedielectric layer 50 such that the dielectric layer 55 covers the end455.

The next stage of the assembly process of the second embodiment is shownin FIG. 22. The through holes 505 of the dielectric layer 50 areextended through the ground plane 73 and the dielectric layer 55.Terminals or contact pads 5513 are then flamed sprayed onto the uppermain surface 552 of the dielectric layer 55, with the terminals 5513 ofthe second embodiment having the same characteristics as the terminals5013 of the first embodiment.

The next stage of the assembly process is shown in FIG. 23. In thisstage, the partially-assembled heater mat of FIG. 22 has a furtherdielectric layer 56 positioned on top of the dielectric layer 55. Twoconnectors 46 (which corresponds to the two connectors 41 of the firstembodiment) and a connector 47 (which corresponds to connector 42 of thefirst embodiment) have their ends 465, 475 brought into electricalcontact with the terminals 5513. A second ground plane 74 is flamesprayed onto the dielectric layer 56 and has characteristicscorresponding to the second ground plane 72 of the first embodiment. InFIG. 23, the dielectric layer 56 does not cover the ends 465, 475 of theconnectors 46, 47. It could, alternatively, be arranged to cover theends 465, 475 and this would, in the end product (the laminated heatermat of the second embodiment), result in slight localised increasedthickness of the heater mat.

The next stage of the assembly process of the second embodiment is shownin FIG. 24. A dielectric layer 57 is positioned on top of the secondground plane 74. A connector 48 (which corresponds to the connector 44of the first embodiment) has an end 485 electrically connected to thesecond ground plane 74. The dielectric layer 57 is shown as having acutout around the end 485. This cutout could be omitted, and thedielectric layer 57 could extend over the end 485.

The next stage of the assembly process is shown in FIG. 25. A furtherdielectric layer 58 is brought into contact with the main surface 502 ofthe dielectric layer 50 so as to cover the heater element 501 and thearea temperature sensor 507. The result is shown in FIG. 26. In FIG. 26,all of the components of the heater mat 3 and the embedded ends 455,465, 475, 485 of the connectors 45, 46, 47, 48 are in position and readyto be laminated together.

Heat and pressure are applied to the stack of components of FIG. 26 toproduce the monolithic laminate of the heater mat 3 shown in FIG. 27.All of the dielectric layers 50, 55, 56, 57, 58 are made of the same orcompatible high-temperature engineering thermoplastic (as per the firstembodiment) and thus fuse together during the lamination process. Wherenecessary, the thermoplastic material flows through the porous groundplanes 73, 74 and through the porous heater element 501 and through theporous temperature sensor 507. Because the thermoplastic material mergesor fuses together at the interfaces between the stacked components ofFIG. 26, the interfaces substantially disappear and thus interfacediscontinuities are, in effect, not present in the end product (theheater mat 3 of the second embodiment). Discontinuities are undesirablebecause they can function as crack initiation sites which are sites ofpotential structural or fatigue weakness. Substantially removingdiscontinuities from the end product (the laminated heater mat 3)produces a more durable heater mat.

The heater mat 3 of the second embodiment (FIG. 27) then has its frontsurface 31 adhesively bonded to the rear surface 142 of the erosionshield 14, as shown in FIG. 28.

The connectors 45, 46, 47, 48 collectively form the bundle of connectors4 which serve to electrically connect the heater mat 3 to the powersupply and control electronics unit 6.

In the second embodiment, the two ground planes (ground planes 73, 74)have different positions relative to the heater element 501 and thetemperature sensor 507 as compared with the two ground planes (groundplanes 71, 72) of the first embodiment.

In the second embodiment, the heater element 501 and the temperaturesensor 507 are not sandwiched between the two ground planes 73, 74.Instead, the two ground planes 73, 74 are positioned on the side of theheater element 501 and temperature sensor 507 remote from the erosionshield 14. In other words, the heater element 501 and the temperaturesensor 507 are sandwiched between (i) the erosion shield 14 and (ii) thetwo ground planes 73, 74. Compared with a heater mat having only asingle ground plane, the two ground planes 73, 74 of the secondembodiment provide improved protection against a lightning strikeinducing excessive currents in the heater element 501, the temperaturesensor 507 and the embedded ends of the connection bundle 4 which leadaway from the heater element 501 and the temperature sensor 507.However, the protection is less effective than the protection providedby the configuration of the two ground planes of the first embodiment,because in the first embodiment the two ground planes 71, 72 sandwichthe heater element 501 and temperature sensor 507 and thus provide atype of “coaxial shielding” to the heater element 501 and temperaturesensor 507.

FIGS. 29 and 30 show an alternative area temperature sensor. In FIG. 29,the area temperature sensor 507 is positioned on a carrier 5019 which isseparate from the dielectric layer 50. The carrier 5019 is of smallerwidth and length than the dielectric layer 50 but is preferably made ofthe same high-temperature engineering thermoplastic as the dielectriclayer 50.

Alternatively but less desirably, the carrier 5019 is made of ahigh-temperature engineering thermoplastic which is compatible with thedielectric layer 50 and the other components of the heater mat 3 withwhich it will be fused during the lamination process. Our currentlypreferred materials for the carrier 5019 include PPS, PEI, PEKK, PEEKand PES. Of these materials, PEKK and PEEK are particularly preferred.

FIG. 29 also shows how the temperature sensor 507 may, optionally, bepartially encapsulated within an encapsulation layer 5020 which is madeof the same material as the carrier 5019. The encapsulation layer 5020is shown in chain-dotted line in FIG. 29. When the encapsulation layer5020 is positioned on the carrier 5019, the encapsulation layer 5020covers all of the sensor head 5010 and the adjacent first parts of theleads 5012.

The area temperature sensor 507 is flame sprayed onto the upper mainsurface 50191 of the carrier 5019. The flame spraying of the temperaturesensor 507 results in the first and second terminals 508, 509 of thetemperature sensor being positioned around through holes 5021 of thecarrier layer 5019.

Then, as shown in FIG. 30, the carrier 5019 is positioned on thedielectric layer 50 of FIG. 3. The positioning is such that the throughholes 5021 of the carrier 5019 align with the relevant through holes 505of the dielectric layer 50.

Other aspects of the manufacturing process for producing a heater matare the same as for the first embodiment described with reference toFIGS. 3-19 or the second embodiment described with reference to FIGS.20-28.

FIG. 31 shows a connector 49 which is a variant of the connector 41 ofFIG. 8.

In relation to the connector 49, it uses the same dielectric substratelayer 411, main surface 412, power track 413, terminal 414 and end 415as for the connector 41 of FIG. 8. The difference is that the connector49 of FIG. 31 additionally includes an encapsulation layer 491 which ismade of a high-temperature engineering thermoplastic the same as orcompatible with the dielectric substrate layer 411. The encapsulationlayer 491 stops at a position 492 of the main surface 412 which leavesexposed the terminal 414 and an adjacent short length of the power track413. The connector 49 may be used to replace the connectors 41, 43 and44 of the first embodiment or the connectors 45, 46 and 48 of the secondembodiment. The position 492 of the connector 49 is chosen so that theend 493 of the encapsulation layer 491 butts up to, and does not enterinto, the laminated components of the heater mat 3.

When the connector 49 is being produced, heat and pressure are appliedto the layers 411, 491 so that they merge or fuse together to form alaminated structure.

However, because the encapsulation layer 491 does not penetrate into thelaminated components of the heater mat 3, it would be possible to changethe material of the encapsulation layer 491 to, for example, aprotective film that is sprayed on. The nature of the material of thesprayed film will not particularly matter in the context of laminatingtogether the components of the heater mat 3, because the material of theencapsulation layer 491 will not penetrate into the stack of componentsforming the heater mat 3.

FIG. 32 shows a further alternative connector 41A which is generally thesame as the connector 41 of FIG. 8, except that a metallic (e.g. copper)plug 416 is attached (e.g. by welding or soldering) to the terminal 414of FIG. 8, thereby to produce the connector 41A as a variant of theconnector 41 of FIG. 8. The plug 416 is shown in FIG. 32 as having acircular base portion 4161 and a circular upper portion 4162 which is ofsmaller diameter than the base portion 4161. The diameter of the upperportion 4162 is preferably set to be the same as that of the holes 505of FIG. 3. Thus, in a variant of FIG. 9, when the connector 41A replacesthe connector 41, the upper portion 4162 of the plug 416 will projectinto and nest neatly inside the corresponding through hole 505 of thedielectric layer 50.

FIG. 33 is a schematic depiction of the connections between the heatermat 3 and the power supply and control electronics unit 6.

The heater mat of the present invention may be incorporated in any (e.g.forwardly-facing) surface of an aircraft that may be prone to iceformation in flight. For example, alternatives to incorporating theheater mat in the leading edge of a wing include incorporating it in theleading edge of a fin or tailplane, or at the air intake of an engine,or in a trailing-edge flap to stop ice formation on the flap when it isdeployed, or in an aileron.

In the above first and second embodiments, the heater mat 3 has beenindependently assembled and then laminated, before being attached to theerosion shield 14. An alternative would be to start with the erosionshield 14 and then stack in sequence, on the erosion shield, thecomponents of the heater mat and the connectors. The first componentcould be bonded to the erosion shield. Then, when the full stack ofcomponents has been assembled onto the first component, heat andpressure could be applied to the components and the erosion shield so asto laminate together the components of the heater mat and the connectorsin situ on the erosion shield.

There have been described first and second embodiments of anelectrothermal heater mat 3 for an ice protection system, wherein: theheater mat 3 is a laminated heater mat and comprises dielectric layers50-58, a heater element 501 and a temperature sensor 507; eachdielectric layer 50-58 comprises thermoplastic material; the temperaturesensor 507 comprises a sprayed metal track 5010, 5012 deposited on asubstrate 50, 5019 comprising thermoplastic material; the substrate 50,5019 is laminated to at least a first one of the dielectric layers 53,58; and the thermoplastic material of the substrate is (i) the same asthe thermoplastic material of the first dielectric layer such that thethermoplastic material of the substrate is dispersed or merged into thethermoplastic material of the first dielectric layer or (ii) compatiblewith the thermoplastic material of the first dielectric layer such thatthe thermoplastic material of the substrate is fused to thethermoplastic material of the first dielectric layer.

There has also been described a method of manufacturing first and secondembodiments of a heater mat 3, comprising the steps of: providing aplurality of dielectric layers 50-58 each comprising thermoplasticmaterial; flame spraying a metal track 5010, 5012 of a temperaturesensor 507 onto thermoplastic material of a substrate 50, 5019; forminga stack comprising the dielectric layers 50-58, a heater element 501 andthe substrate 50, 5019; and laminating together the dielectric layers50-58 and the substrate 50, 5019 such that the thermoplastic material ofthe substrate (i) disperses or merges into or (ii) is fused to thethermoplastic material of the or each adjacent one of the dielectriclayers 51, 53, 55, 58.

The invention claimed is:
 1. An electrothermal heater mat for an iceprotection system, wherein: the heater mat is a laminated heater mat andcomprises dielectric layers, a heater element and a temperature sensor;each dielectric layer comprises thermoplastic material; the temperaturesensor comprises a sprayed metal track deposited on a substratecomprising thermoplastic material; the substrate is laminated to atleast a first one of the dielectric layers; and the thermoplasticmaterial of the substrate is (i) the same as the thermoplastic materialof the first dielectric layer such that the thermoplastic material ofthe substrate is dispersed or merged into the thermoplastic material ofthe first dielectric layer or (ii) compatible with the thermoplasticmaterial of the first dielectric layer such that the thermoplasticmaterial of the substrate is fused to the thermoplastic material of thefirst dielectric layer.
 2. An electrothermal heater mat according toclaim 1, wherein the substrate is a second one of the dielectric layers.3. An electrothermal heater mat according to claim 2, wherein thesprayed metal track of the temperature sensor is porous and thethermoplastic material of the first dielectric layer is laminated to thethermoplastic material of the second dielectric layer through thesprayed metal track of the temperature sensor.
 4. An electrothermalheater mat according to claim 2, wherein the heater element comprises asprayed metal track deposited on the second dielectric layer.
 5. Anelectrothermal heater mat according to claim 1, wherein the substrate isa carrier which has a main surface which is smaller than a main surfaceof a second one of the dielectric layers onto which the carrier islaminated, and the carrier is sandwiched between the first and seconddielectric layers.
 6. An electrothermal heater mat according to claim 5,wherein the carrier covers 10% or less of said main surface of thesecond dielectric layer.
 7. An electrothermal heater mat according toclaim 5, wherein the sprayed metal track of the temperature sensor isporous and the thermoplastic material of the first dielectric layer islaminated to the thermoplastic material of the carrier through thesprayed metal track of the temperature sensor.
 8. An electrothermalheater mat according to claim 5, wherein the heater element comprises asprayed metal track deposited on a part of the second dielectric layernot covered by the carrier.
 9. An electrothermal heater mat according toclaim 1, wherein the sprayed metal track of the temperature sensorcomprises a sensor head and the metal of the sensor head is nickel ornickel alloy.
 10. An electrothermal heater mat according to claim 9,wherein the sensor head extends between intermediate positions of thesprayed metal track of the temperature sensor, and the sprayed metaltrack of the temperature sensor further comprises leads which extendfrom the intermediate positions to terminals at the ends of the sprayedmetal track.
 11. An electrothermal heater mat according to claim 9,wherein the substrate is a carrier which has a main surface which issmaller than a main surface of a second one of the dielectric layersonto which the carrier is laminated, and the carrier is sandwichedbetween the first and second dielectric layers and an encapsulationlayer comprises the same thermoplastic material as the thermoplasticmaterial of the carrier, and the encapsulation layer is laminated to thecarrier and covers the sensor head of the temperature sensor.
 12. Anelectrothermal heater mat according to claim 1, wherein the or eachthermoplastic material comprises PEEK, PEKK, PPS, PEI or PES or amixture thereof.
 13. An electrothermal heater mat according to claim 1,wherein the or each thermoplastic material comprises PEEK, PEKK or amixture thereof.
 14. An electrothermal heater mat according to claim 1,wherein the thermoplastic material of the substrate is the same as thethermoplastic material of the dielectric layers.
 15. An electrothermalheater comprising a heater mat according to claim 1 and at least oneconnector having a first end which is electrically connected to theheater element and a second end which extends away from the heater matfor connection to a heater control unit.
 16. An electrothermal iceprotection system comprising an electrothermal heater according to claim15 and a heater control unit to which the second end of the or eachconnector is connected.
 17. Ice protected apparatus comprising anexternal skin and an electrothermal heater according to claim 15,wherein the heater mat is in thermal contact with a rear surface of theexternal skin.
 18. A nose skin comprising an erosion shield and anelectrothermal heater according to claim 15, wherein the heater mat isbonded to a rear surface of the erosion shield.
 19. A wing slatcomprising a rear section and a forward section comprising a nose skinaccording to claim
 18. 20. A wing slat according to claim 19, whereinthe nose skin is demountable from the rear section.
 21. A method ofmanufacturing an electrothermal heater mat, comprising the steps of:providing a plurality of dielectric layers each comprising thermoplasticmaterial; flame spraying a metal track of a temperature sensor ontothermoplastic material of a substrate; forming a stack comprising thedielectric layers, a heater element and the substrate; and laminatingtogether the dielectric layers and the substrate such that thethermoplastic material of the substrate (i) disperses or merges into or(ii) is fused to the thermoplastic material of the or each adjacent oneof the dielectric layers.
 22. A method according to claim 21, whereinthe substrate is a first one of the dielectric layers and the methodfurther comprises the step of flame spraying the heater element on saidfirst dielectric layer.
 23. A method according to claim 21, wherein thesubstrate is a carrier which has a main surface which is smaller than amain surface of a first one of the dielectric layers onto which thecarrier is laminated during the laminating step, and the carrier issandwiched between said first dielectric layer and a second one of thedielectric layers during the stack forming step.
 24. A method accordingto claim 23, further comprising the step of flame spraying the heaterelement on a part of said first dielectric layer not covered by thecarrier.
 25. A method according to claim 21, wherein the or eachthermoplastic material comprises PEEK, PEKK, PPS, PEI or PES or amixture thereof.
 26. A method according to claim 21, wherein the or eachthermoplastic material comprises PEEK, PEKK or a mixture thereof.
 27. Amethod according to claim 21, wherein the thermoplastic material of thesubstrate is the same as the thermoplastic material of the dielectriclayers.